Light emitting panels are commonly used as backlights for display systems (e.g, liquid crystal displays), although they may be used for other purposes as well. Light emitting panels commonly used for backlighting displays include cold cathode florescent lamps (CCFL) and electro-luminescent panels. These types of light emitting panels are advantageous in that they provide substantially uniform illumination over the entire area of the panel, thus providing even illumination for the display. Unfortunately, however, CCFL and electro-luminescent panels are not without their disadvantages, including poor color rendition. In addition, they may involve the use of hazardous materials, such as mercury.
Partly in an effort to address the shortcomings of CCFL and electro-luminescent panels, light emitting panels have been developed that utilize light emitting diodes (LEDs) as the light sources. Besides being mercury-free, LED light emitting panels typically provide better color rendition than CCFL and electro-luminescent panels.
One type of LED light emitting panel is the edge-lit panel. In an edge-lit panel, a plurality of LEDs are positioned adjacent one or more of the edges of a light guide panel. Light from the LEDs enters the edges of the light guide panel and is re-directed within the light guide panel so that the light emerges from the front face of the light guide panel. A reflector may be provided on the back surface of the panel to reflect light toward the front surface of the panel that would otherwise exit from the back surface of the panel.
Unfortunately, edge-lit light emitting panels are not without their drawbacks. For example, the placement of the LEDs along the edges of an edge-lit panel limits the ability to dissipate heat produced by the LEDs. The edge-lit configuration also limits the maximum size of the panel, in that the number of LEDs that may be used to illuminate the panel increases linearly with the edge length, but the area that must be illuminated increases as the square of the edge length. Consequently, edge-lit panels are typically limited to panels having small areas.
Another type of LED light emitting panel is a so-called back-lit panel in which a plurality of LEDs are arranged in a two-dimensional array adjacent the back surface of the panel. The panel diffuses (i.e., evens-out) the light from the LEDs so that the panel appears to provide more even illumination than would be possible with just the LEDs alone. While back-lit panels do not suffer from the brightness limitations of edge-lit designs, and they may be used with panels having larger areas, it has proven difficult to effectively diffuse the light from the individual LEDs so that the panel appears to be evenly illuminated.
For example, one way to improve the illumination uniformity of a back-lit LED panel is to place the LEDs closer together. Disadvantageously, however, this increases the cost of the panel as more LEDs must be used. The closer spacing of the LEDs can also create heat dissipation problems. Another way to improve the illumination uniformity is to cause the panel to provide increased light diffusion. However, increased light diffusion typically represents a decrease in efficiency, thereby reducing the brightness of the panel or requiring the use of more or brighter LEDs to compensate for the efficiency loss associated with the increased diffusion.